Abstract
High-temperature-resistant fiber Bragg gratings (FBGs) are the main competitors to thermocouples as sensors in applications for high temperature environments defined as being in the 600–1200 °C temperature range. Due to their small size, capacity to be multiplexed into high density distributed sensor arrays and survivability in extreme ambient temperatures, they could provide the essential sensing support that is needed in high temperature processes. While capable of providing reliable sensing information in the short term, their long-term functionality is affected by the drift of the characteristic Bragg wavelength or resonance that is used to derive the temperature. A number of physical processes have been proposed as the cause of the high temperature wavelength drift but there is yet no credible description of this process. In this paper we review the literature related to the long-term wavelength drift of FBGs at high temperature and provide our recent results of more than 4000 h of high temperature testing in the 900–1000 °C range. We identify the major components of the high temperature wavelength drift and we propose mechanisms that could be causing them.
Highlights
The majority of present-day industrial processes require complex monitoring and control
Three different types of gratings were tested, each of them being a Type II fiber Bragg gratings (FBGs) made with different exposure condition and resulting in a different grating structure as presented above in Section 3.3, namely: (1) Type-a gratings made with 80 fs pulses (Fourier-transform-limited pulses) and a first-order phase mask; (2) Type-b gratings made with 400 fs pulses and a third-order phase mask; (3) Type-c gratings made with 80 fs pulses (Fourier-transform-limited pulses) and a third-order phase mask
For FBGs to have a future as temperature sensors in the 900–1000 ◦C range, the phenomenon of Bragg wavelength drift needs to be addressed
Summary
The majority of present-day industrial processes require complex monitoring and control. The main disadvantages of thermocouples that currently limit their use are their large size (the electromotive force is essentially generated by thermal gradients along the entire length of the thermocouple), redistribution of impurities/dopants along the thermocouple length at high temperatures (the electromotive force is altered during the measurements) and sensitivity to electromagnetic interference and ionizing radiation [1]. These drawbacks can potentially be addressed by fiber optic sensors (FOSs). There are experiments described in the literature that involve simultaneous measurements by FOSs and thermocouples [3,4,5] and show similar temperature measurement results
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